From the Guidelines

The stage of left-sided breast cancer with contralateral lymph node metastasis, but no distant metastasis, is M1.

The presence of contralateral lymph node metastasis is considered distant metastasis according to the AJCC staging system 1.
The AJCC staging system defines distant metastasis (M1) as the presence of metastases in distant organs or lymph nodes, including contralateral lymph nodes 1.
The TNM staging system is used to classify breast cancer, with T describing the size of the primary tumor, N describing the extent of lymph node involvement, and M describing the presence of distant metastasis 1.
The classification of lymph node involvement is critical in determining the stage of breast cancer, with different categories (N0, N1, N2, N3) describing the extent of lymph node involvement 1.
The presence of contralateral lymph node metastasis would classify the cancer as stage IV, as it indicates distant metastasis 1.

The AJCC staging system is the standard system used to classify breast cancer, and it takes into account the size of the primary tumor (T), the extent of lymph node involvement (N), and the presence of distant metastasis (M) 1.
The staging system is as follows:
- Stage I: T1, N0, M0
- Stage II: T0-2, N1, M0 or T2, N0, M0
- Stage III: T0-3, N2-3, M0 or T3, N1-2, M0
- Stage IV: Any T, Any N, M1
The presence of contralateral lymph node metastasis would classify the cancer as stage IV, as it indicates distant metastasis 1.

From the Research

The stage of left-sided breast cancer with contralateral (opposite side) lymph node metastasis, but no distant metastasis, is a topic of debate among researchers and clinicians 2, 3, 4, 5, 6.
According to the current TNM system, contralateral axillary lymph node metastasis (CAM) is classified as M1, stage IV disease 2, 5.
However, several studies suggest that CAM should be considered a locoregional event rather than a distant metastasis, and therefore, should be classified as stage III disease 2, 3, 4, 5, 6.
The prognosis of patients with isolated CAM is better than that of patients with other distant metastases, with a 5-year survival rate of 67.4% and a 5-year disease-free survival rate of 52.9% 4.
Treatment strategies for CAM, including axillary lymph node dissection and radiotherapy, have been shown to improve overall survival and disease-free survival in patients with isolated CAM 4, 5, 6.

The classification of CAM as a locoregional event rather than a distant metastasis has implications for treatment and staging of breast cancer patients 2, 3, 4, 5, 6.
Patients with CAM may benefit from aggressive and curative treatment strategies, rather than palliative care 4, 5, 6.
Further research is needed to determine the optimal treatment approach for patients with CAM and to revise the current TNM staging system to reflect the unique characteristics of this clinical entity 2, 3, 4, 5, 6.